Windows NT was Microsoft's first server operating system. (The NT stands for New Technology, which means that the Windows 2003 Server slogan "Built on NT Technology" expands to "Built on New Technology Technology")
DOS debuted in 1981 for use in personal computers. As time went on, despite frequent enhancements including, most notably, its extension into a 32-bit system with a semi-integrated GUI for DOS 7.0 (Windows 95), it was felt by IBM and Microsoft to be rather lacking in scalability. Thus the two companies began working on OS/2.
After a falling out, IBM was left to introduce OS/2 and witness it fail due to marketing reasons, while Microsoft hired Dave Cutler (a major player in VMS) to help them design NT. This was originally to be a "New Technology" but, with the increasing success of Windows, it became "Windows NT". (Windows was something of a commercial failure until Windows 3.0.) Rumour says that it is no coincindence that VMS and WNT are just one letter separated (V-W, M-N, S-T).
So, in 1993, the first version, NT 3.1 (given a version number to match the current Windows add-on to DOS) was released. It used the basic Windows GUI but, underneath, was a 32-bit, multi-tasking, multi-user, networking operating system. With the smashing success of Windows 95 (Windows 4.0 with DOS 7.0 underneath), NT adopted that GUI. Windows 98 was the last genuine DOS/Windows system. NT 5.0 was released as "Windows 2000" and replaced the original Windows line, which was killed off with the "Windows ME" release. Currently, NT 5.1, released as "Windows XP", is Microsoft's main desktop OS offering and NT 5.2, released as "Windows Server 2003", is their current Server platform.